I cleaned the water level indicator with hot soapy water and washing up brush. I had to do it from the sump as I couldn't get the component out due to the water heater in the way. I didn't really want to dismantle that!
I could hear the microswitch clicking away as I cleaned it, so I imagine it's moving now. This did not fix the problem.
I then fetched off the left side panel this morning at 7am (I couldn't sleep, thinking about all the washing up I might have to do!). I disconnected the connector to the flow meter. It looked clean, but I gave it a going over. I also cleaned the tabs on the mini PCB. Reconnected and tried a rinse.
Drain, drain, drain - WATER! Ah HA! It's working!
I shone the torch at the water wheel and I could see it start and stop spining with the water flow.
I did notice a spot of rust on the lower support, underneath the water wheel. I did find moisture on my fingers from the wheel area. I wonder if this is condensation and not a leak?
So, tonight, when I get home from work I will run a normal wash a see if it will run normally.
Thank you, thank you, thank you.
Note - my hands are cut to hell. If you don't like drawing blood for whatever reason then wear some protection on your hands. A lot of stuff in the base of my Whirlpool dishwasher has very sharp edges.